Glucosanine 6-phosphate deaminases from E. coli and from mammalian species are allosteric enzymes. The current knowledge about the kinetics indicate that this enzyme may be a useful system for the study of allosteric transitions. Moreover the human enzyme has a.a. identity of 50% in relation to the E. coli enzyme, is 23 a.a. longer at its C-terminus, and changes drastically its allosteric properties, from a K-pure system in E. coli to a V-pure system in human. We have measured at Stanford, the diffraction data of the human R-conformer (77% of the data at 2.4 E resolution). The crystal have a hexamer (1734 residues) in the asymmertic unit. The refinement is under process but with the available data we found only 8 of the 23 a.a. at the C-terminal. In the E.coli T structure that we measured at SSRL this year we detected that the active-site lid has large B-factors which is associated to an enthropic term associated with the allosteric transition. This is the first time that structural data on an allosteric enzyme relates atom mobility to regulation properties and we plan to measure this crystals an other complexes of this enzyme to higher resolution to evaluate anisotropic B factors to relate specific vibrational modes to this entropic allosteric effect.